An important mechanism regulating tissue perfusion is vasodilation in response to flow-induced shear stress. In most blood vessels shear induces formation of a compound within the endothelium, which is transferred to the underlying vascular smooth muscle to produce vasodilation. The principal mechanism involves endothelial production and release of nitric oxide. Recent preliminary data show that human coronary arterioles demonstrate prominent flow-induced relaxation which is independent of nitric oxide synthase. However, presence of potassium chloride does prevent flow-mediated dilation. The specific goal of this application is to determine the mechanism of human coronary arteriolar dilation to changes in flow. It is hypothesized that contrary to most studies performed in animals, flow-induced dilation of human coronary arterioles is not due to release of nitric oxide, rather an endothelial- derived hyperpolarization factor plays an important role. This application will examine the role of the endothelium, contribution of membrane hyperpolarization, determine the critical involvement of potassium channels, examine the novel hypothesis that free-radicals mediate flow-induced dilation and determine whether specific metabolites of arachidonic acid (epoxyeicosotrienoic acids) are required for flow- induced vasodilation in human coronary microvessels. Furthermore, since free radical stress in atherosclerosis reduces NO dilation, the hypothesis will be tested that flow-mediated dilation (not NO-mediated) is not altered in microvessels from patients with atherosclerosis; however, dilation to ADP (NO-mediated) in the presence of flow is impaired. Fresh human coronary arterioles isolated from myocardial tissue obtained at the time of cardiopulmonary bypass are mounted intact on micropipettes and pressurized for diameter measurements using in vitro videomicroscopy. Simultaneous measurements of vascular smooth muscle membrane potential and diameter in cannulated human arterioles will be made. This combination of pharmacological and electrophysiological assessment of vascular function provides a powerful mechanism for studying endothelium-dependent regulatory processes in health and disease. These studies will have important implications with regard to coronary vasomotor regulation. Coronary arterioles responsible for regulating myocardial perfusion (between 50-150 microns in diameter) will be studied. Results from the proposed experiments should identify in human subjects novel mechanisms of coronary vasoregulation and explain how these may be altered in atherosclerosis. The results could suggest new therapeutic approaches for the treatment of microvascular dysfunction in patients with coronary disease.